Our invention relates generally to vehicle acceleration slip control for maintaining maximum frictional driving force at the vehicle drive wheels, the frictional force being determined by the loading on the drive wheels and the coefficient of friction between the road surface and the drive wheels during operation of the vehicle.
We are aware of prior art traction control systems using electronic microprocessors for limiting the tractive effort developed by vehicle traction wheels at the time of vehicle acceleration. A typical design approach is embodied in the acceleration slip control system of U.S. Pat. No. 4,554,990 in which a drive wheel speed sensor senses rotational speed of the drive wheel to produce a first speed signal and a separate speed sensor detects the speed of the non-driving or driven wheel. The two signals are compared to determine whether a slip condition exists. If the speed of the driven wheel at any given instant is less than the speed of the drive wheel, a control signal representative of the magnitude of the slip is generated.
A microprocessor in the system described in the '990 patent determines the rate of change of angular velocity of the drive wheel with respect to the driven wheel in order to provide the input data for an acceleration calculation performed by the microprocessor. A drive wheel controller responds to the control signal that represents drive wheel slip for controlling the driving force of the wheel. This usually is accomplished by changing the engine torque so that a controlled amount of slip can be maintained to achieve maximum tire traction.
It is desirable to maintain a small degree of slip, measured as a percentage of wheel speed, in order to achieve maximum traction control. Typical examples for desired wheel slip for maximum traction might be some value between 3% and 8% of driven wheel speed.
Another prior art teaching dealing with acceleration slip control is found in U.S. Pat. No. 4,788,644 wherein provision is made for maintaining a controlled slip of approximately 10% of the driving wheel velocity using driving wheel speed and non-driving wheel speed as sensed input data for a microprocessor, and wherein provision is made for terminating the slip control to enable immediate acceleration when the need for traction control ceases to exist during the acceleration mode.
In prior art teachings such as those found in the prior art patents mentioned above, the management of the traction control torque includes a controller that reduces engine torque either directly or by means of an electronic signal command to an electronic microprocessor, which in turn controls the engine fuel delivery rate or fuel air ratio or some other engine control parameter. Such a system, of necessity, results in control errors due to torque friction torque and other torque losses including transmission gearing losses as well as unknown torque multiplication developed by the transmission gearing and by the hydrokinetic torque converter in the torque delivery driveline.